Presence detector

ABSTRACT

An electronic system for detecting the presence of an external object in the field of one of the reactors in the tank circuit of the first one of a pair of oscillators. The oscillators are interconnected for frequency synchronization. The tank circuits of the oscillators are tuned to have different natural resonant frequencies in the absence of an object whereby the oscillator signals differ in phase. The presence of an object affects the tuned resonant frequency of the tank circuit of the first oscillator in a direction or sense to decrease the phase difference between the oscillator output signals. The oscillator output signals are limited, fed to a phase detector, and a signal is developed representing the phase differential. When this signal decreases below a predetermined threshold, an output indicator is triggered to provide an indication of the presence of the external object. The threshold is established in part by a capacitor having a charge representing ambient conditions when no object is present. The charging circuit for the capacitor provides a short time constant to follow gradually changing ambient conditions but provides a long time constant to prevent change in the charge on the capacitor as the result of the relatively more rapid and large change in the impedance of the reactance element as an external object arrives within the detection field. The long time constant circuit is operative regeneratively to change the charge on the capacitor after a long delay following the detection of an arriving object in case the object has not in the interim departed from the field. The departure of the object from the field prior to the time that the charge has leaked from the memory capacitor, returns the capacitor charge to ambient condition. The system is particularly adapted to indicate presence of vehicles within the field of a loop disposed slightly below the surface of a roadway or driveway.

nited States Patent Wilcox [54] PRESENCE DETECTOR Merton F. Wilcox, 3135N. Washington Blvd., Sarasota, Fla. 33580 [22] Filed: Jan. 7, 1970 [21]Appl.No.: 1,231

[72] Inventor:

[52] US. Cl. ..340/258 C, 340/258 A [51] Int. Cl. ..'.G08b 13/00 [58]'Field of Search ..340/258 C, 38 L, 258 A [56] References Cited UNITEDSTATES PATENTS Primary ExaminerJohn W. Caldwell AssistantExaminerMichael Slobasky Attorney-George H. Baldwin and Arthur G. Yeager[57] ABSTRACT An electronic system for detecting the presence of anexternal object in the field of one of the reactors in the tank circuitof the first one of a pair of oscillators. The oscillators areinterconnected for frequency Aug. 8, 1972 synchronization. The tankcircuits of the oscillators are tuned to have different natural resonantfrequencies in the absence of an object whereby the oscillator signalsdiffer in phase. The presence of an object affects'the tuned resonantfrequency of the tank circuit of the first oscillator in a direction orsense to decrease the phase difference between the oscillator outputsignals. The oscillator output signals are limited, fed to a phasedetector, and a signal is developed representing the phase differential.When this signal decreases below a predetermined threshold, an outputindicator is triggered to provide an indication of the presence of theexternal object. The threshold is established in part by a capacitorhaving a charge representing ambient conditions when no object ispresent. The charging circuit for the capacitor provides a short timeconstant to follow gradually changing ambient conditions but provides along time constant to prevent change in the charge on the capacitor asthe result of the relatively more rapid and large change in theimpedance of the reactance element as an external object arrives withinthe detection field. The long time constant circuit is operativeregeneratively to change the charge on the capacitor after a long delayfollowing the detection of an arriving object in case the object has notin the interim departed from the field. The departure of the object fromthe field prior to the time that the charge has leaked from the memorycapacitor, returns the capacitor charge to ambient condition. The systemis particularly adapted to indicate presence of vehicles within thefield of a loop disposed slightly below the surface of a roadway ordriveway.

5 Claims, 2 Drawing Figures PATENTEDAus a 1912 SHEET .1 BF 2 R O T N E VW PRESENCE DETECTOR This invention relates to detectors responsive tothe presence of an object in the field of a reactance element. Objectshaving electrical or magnetic characteristics such that the impedance ofthe reactance element is affected by the presence of the object in thefield are detected in accord with the invention. While the invention hasutility for detecting various types of objects, it is particularlyuseful in detecting the presence of paramagnetic or conductive objects,such as vehicles, which arrive within the field of a detecting loopdisposed typically an inch or two below the surface of a roadway ordriveway or the like, and the system specifically illustrated anddescribed herein is particularly adapted for detecting and indicatingthe presence of automobiles, bicycles and other vehicles within thefield of such loop.

The system described herein embodies two oscillators tuned to differentresonant frequencies but so interconnected as to remain in frequencysynchronization. The detecting loop is incorporated in the tank circuitof one of the oscillators which is tuned to resonance at a lowerfrequency than that of the other oscillator. The signals from the twooscillators are applied to a phase detector. The presence of anautomobile tunes the loop circuit to a higher frequency than that atwhich it resonates in the absence of the automobile, and the phasedifference between the signals is reduced, changing the phase detectoroutput. The phase detector output is measured against the charge on amemory capacitor, and, upon an abrupt change in such output, anindicator is actuated. The memory capacitor charge is causedcontinuously to represent ambient conditions in the system, which maychange slowly with change in temperature of the loop, for example.

The novel features which are believed to be characteristic of thisinvention are set forth with particularity in the appended claims. Theinvention itself, however, both as to its organization and method ofoperation, together with further objects and advantages thereof, maybest be understood by reference to the following description taken inconnection with the accompanying drawings, in which:

F IG. 1 is a circuit diagram of a detector system embodying theinvention, and

FIG. 2 is a set of voltage curves illustrating the tuning and operationof the systems.

In FIG. 1, the detector system is shown as comprising a buried loop 1 ina vehicle trafiic lane 2, positioned to detect the presence of anexternal object in the form of vehicle 3 which may pass into or bepresent within the field of the reactance element in the form of aninductance loop 1. In this environment, the loop 1 may comprise twoturns in a configuration extending four feet along and nine feet acrossthe vehicle traffic lane and connected by 50-foot leads 4 and 5 to thedetector circuit which is, accordingly, conveniently located remote fromthe position of the buried loop. While the loop is represented as arectangular loop with long legs extending perpendicularly across thelane and short legs parallel thereto, other shapes ad forms of loops maybe employed, which may be circular or oval, or in the shape ofparallelograms, triangles or other geometric configurations, with legswhich extend perpendicularly or diagonally with respect to the directionof vehi cle travel. Reactance element 1 is connected in parallel withreactance elements 6 and 7 which are connected in series. Elements 6 and7 in the circuit shown comprise capacitors constituting with inductanceloop 1 a frequency'determining circuit of an oscillator generallyidentified by the numeral 8. The circuit of inductance loop 1 andcapacitors 6 and 7 comprises a tank circuit. The oscillator includes aPNP-transistor 9 having its emitter connected through resistor 10, whichmay be 470 ohms, to positive bus 11, and with its base connected to thejuncture of voltage divider resistors 12 and 13 which are connected inseries from positive bus 11 to negative bus 14. Resistor 12 connectedbetween the base and the positive bus may be of lOK ohms while resistor13 connected from the base to the negative bus may have a resistance of3.3K ohms. One of the leads 4 from the loop 1 is connected to collector15 of transistor 9 while the other lead 5 is connected to the negativebus. A network comprising comprising Zener diodes l6 and 30, each ofwhich may be of type 1N757, and resistors 17 and 18, each of 0.1'megohm, is connected between the negative bus and the collector oftransistor 9. The cathodes of the diodes are interconnected andconnected to the juncture of the serially connected resistors 17 and 18,while one resistor is connected at its free end to the collector 15 andthe other resistor 18 is connected at its free end to the negative bus.The anode of one diode is connected to the collector l5 and the anode ofthe other to the negative bus.

Oscillator 8 is arranged as a Colpitts oscillator, with the emitter oftransistor 9 connected by a lead 19 to the midpoint or juncture ofcapacitors 6 and 7, each of which may be 0. 1 rnfd.

A second Colpitts oscillator is identified at 20 and comprises atransistor 21 which, like transistor 9, may be of type 2N3906. The baseof transistor 20 is connected to the juncture of resistors 22 and 23which are connected in series from the positive to the negative bus.These resistors correspond respectively to resistors 12 and 13 ofoscillator 8 and may be of the same respective resistance values. Thecircuit for collector 24 of transistor 21 comprises a variableinductance 25 connected between the collector and the negative bus andconnected in parallel with series connected capacitors 26 and 32 to forma tank circuit. Capacitors 26 and 32, like capacitors 6 and 7, may be of0. l mid, and the mid-point between these capacitors is connected bylead 27 to the emitter of transistor 21. The emitter is furtherconnected through a 1,000 ohm resistor 28 to the positive bus. Theoscillators are coupled by a coupling impedance in the form of a commonload resistor 29, which may be of 470 ohms, and which is connectedbetween collectors l5 and 24. The resistor 29 constitutes a drive meansinterconnecting the oscillators and locking the oscillators intofrequency synchronization.

The tank circuits of the respective oscillators when the system isoperative and balanced for appropriate operation are tuned to naturalresonant frequencies which are slightly different.

The output signal from oscillator 8 appearing on collector 15 and on therespective end of resistor 29 connected thereto is fed through resistor33, which may be of 1,000 ohms, to the base 34 of phase detectortransistor 35, while the output signal from oscillator 20, appearing oncollector 24 and at the opposite end of resistor 29, is supplied throughresistor 36, which may be of 100 ohms, to the emitter 37 of transistor35. Transistor 35 is of the NPN-type 2N3904.

Reverse connected diodes 38 and 39 are connected between the negativebus and base 34 of transistor 35, while identical reverse connecteddiodes 40 and 41 are in the same manner connected between the emitter 37and the negative bus. These diodes act as clippers or limiters wherebythe respective signals applied to the base and emitter of the transistor35 are square waves of identical amplitudes.

The collector 42 of transistor 35 is connected through load resistor 43to the positive bus and to anode 44 of diode 45. The diode is connectedto provide negative going pulses to the base 46 of transistor 47, ofwhich the collector 48 is directly connected to the negative bus and theemitter 49 is connected through resistor 50 to the positive bus.

Connected between the base 46 and the positive bus is a 0.1 megohmresistor 51 bypassed by a 0.1 mfd. capacitor 52. Each of diodes 38, 39,40, 41 and 45 is of type 1N9l4.

The signal appearing on emitter 49 of transistor 47 is applied through a100-ohm resistor 53 to a large memory capacitor 54 of 1,000 mfd.Capacitor 54 is connected between resistor 53 and the base 55 of atransistor 56, while a resistor 57 is connected between base 55 and thenegative bus. The value of resistor 57 determines the discharge timeconstant for capacitor 54. Thus, if an object arrives over the loop andis detected but then does not leave and remains over the loop, thecharge on capacitor 54 leaks through resistor 7 at a rate determined bythe value of the resistor until the potential across the resistor isreduced to a value at which the presence signal disappears. A typicalvalue for resistor 57 is meg and, if capacitor 54 has a capacity of1,000 mfd., the signal would disappear approximately 30 minutes after anobject 3 had arrived over loop 1 if the object then remained over theloop. If it is desired that the signal should disappear in a shortertime, the value of resistor 57 may be made less, as for example, 1megohm, providing a much shorter period for a presence signal.

An additional connection to the negative bus is provided for base 55through 330K ohm resistor 57, collector 49 and emitter 62 of transistor60 and from emitter 62 through a 10K ohm resistor 61, all in series.

This circuit, which parallels resistor 57, provides, when transistor 60is conductive, a shorter time constant for capacitor 54 than does theresistor 57 alone. The base of transistor 60 is connected through 0.47megohm resistor 63 to the collector 64 of transistor 56. The emitter 65of transistor 56 is directly connected to the positive bus. Transistor56 under ambient conditions, in the absence of an object in the field ofloop 1, is in saturation, base current being supplied through resistor57 and through the collector emitter circuit of transistor 60.Transistor 60 is, under ambient conditions, maintained in saturation bythe positive voltage on collector 64 as developed across collectorresistor 66. A small capacitor 67 of 1,000 pf is connected from base tocollector of transistor 56, while resistor 63 is bypassed by a 10K ohmresistor 68 and 8 mfd. capacitor 69 connected in series. Resistor 66 istypically of 1 8K ohms.

The collector 64 of transistor 56 is connected through resistor 70 of10K ohms and through resistor 71 of 0.47 megohms bypassed by 8 mfd.capacitor 72 to the base 73 of transistor 74. Transistors 60 and 74 areNPN-type 2N3904, while transistors 47 and 56 are PNP-type 2N3906 as istransistor 75. The base 76 of transistor is connected to the collector77 of transistor 74K through resistor 78 of 3.3K ohms. The emitter 79 oftransistor 74 is connected through Zener diode 80 to the negative bus,this diode being of type 1N703 and providing a constant emitter voltageof 3.1 volts with respect to the negative bus. Resistor 81 of 27K ohmsconnects the juncture of resistor 78 and base 76 to a positive voltagesupply line 82 normally maintained at substantially 30 volts positivewith respect to the negative bus, and the emitter 83 is connected to thevoltage supply line through a 100 ohm resistor 84. The collector 85 oftransistor 75 is connected through relay coil 86 to the negative bus,the relay coil being bypassed by a diode 87, also of type 1N9 14.

The voltage drops in high impedance resistors 63 and 71 are such that,while large signals received on base 55 of transistor 56, such assignals generated in response to presence of an automobile over loop 1,are sufficient to cutoff transistors 60 and 74, small signals generatedin response to arrival of a bicycle over the loop, for example, would beinsufficient to cause such cut off were these resistors the onlyconnection to the bases of transistors 60 and 74. Low impedancecapacitor 69 bypassing resistor 63, however, causes transistor 60 to cutoff momentarily when a small signal is applied through capacitor 54,such as in response to the arrival of a bicycle over the loop, and lowimpedance capacitor 72 applies such a signal to cut off transistor 74,thereby providing a momentary actuation of relay 86, 8

A larger signal, corresponding to presence of an automobile, issufficient, as previously described, to hold transistors 60 and 74 innon-conductive condition until the automobile leaves the vicinity of theloop or until the signal leaks from capacitor 54 through resistor 57,whichever first occurs. It will be understood that the signal applied tocapacitor 54 when an automobile leaves the loop vicinity is of oppositepolarity to that applied when an automobile arrives at the loop, andthat such signal applied upon such leaving of the vicinity causestransistors 56 and 60 to become, or to remain, conductive.

Transistor 56 is, under ambient conditions, saturated, and the positivevoltage which exists on its collector during ambient conditionsmaintains transistor 60, and thus transistor 74 and transistor 75, insaturation. Under these conditions relay coil 86, which is preferablybypassed by an 8 mfd. capacitor 31, carries current, holding in relayarmature 88, that is, with the armature in contact with switch terminal89. With the relay thus energized, a circuit is completed through anydesired indicator means, such as indicator lamp connected in series withthe battery or other power source 91. Drop out of the relay causesarmature 88 to drop into contact with terminal 92, opening the circuitthrough output indicator 90 and energizing output indicator 93. It willbe understood that the indicator means 90 and 93 are exemplary only andthat the relay 86, 88, 89, 92 may be employed to operate countingmechanisms, recording mechanisms, traffic control equipments, ticketspitters, or other known devices which it may be desired to operate inresponse to the presence or absence of a vehicle or other object at theloop location. Any of such devices are intended to be included withinthe meaning of the term indicator as used herein.

Power is supplied for the unit from a suitable alternating currentsource 94 through transformer 95 and full wave diode rectifier 96developing approximately 30 volts d.c. across capacitor 97 for supplyingthis positive potential to line 82. The collector-emitter circuit ofvoltage regulating transistor 98 is connected in series to the positivebus 11 to provide regulated volts d.c. potential thereto. Transistor 98is controlled in a known manner by transistors 99 and 100 in response tovariations from 20 volts as detected by Zener diodes 101 and 102. It isfound preferable that the negative bus be not directly connected to anexternal ground and, accordingly, a small NE2 neon lamp 103 is shown asconnected from the negative bus to an external ground 104.

The arrangement as shown and described herein is particularly useful toindicate the absence or presence of automobiles as external objects overa buried, or if desired, a ground level loop 1, and in a typical case apassenger car, with a 4 foot by 9 foot two-tum loop with 50-foot leads 4and 5 will provide a 3-volt change in the voltage developed acrossresistor 50, while a smaller object such as a' bicycle crossing such aloop may produce a change under these conditions of somewhat less than 1volt.

The arrangement is tuned for properoperation by adjusting variableinductance to tune the tank circuits of oscillators 8 and 20 to have thesame natural resonant frequency. A representative satisfactory frequencyis 50 kilocycles. Should it be found necessary a small additionalcapacitance may be connected across the loop terminals to reduce theresonant frequency of oscillator 8 or across the terminals of coil 25 toreduce the resonant frequency of oscillator 20. With reference to FIG. 2of the drawings, when the tank circuits are so adjusted to have the sameresonant frequencies, the result is that in-phase signals of equalamplitudes are applied to the base 34 and emitter 36 of peak detectortransistor 35, as represented, respectively, by curves 134 and 136 inthe portion of FIG. 2 identified by the legend Tuned to Zero Output. Theresultant voltage appearing on the collector 42 of the peak detector isrepresented as the constant voltage of curve 142. Under these conditionsthe voltage appearing across resistor 50 in the emitter circuit oftransistor 47 will be zero volts. It will be understood that the curve134 represents the output of oscillator 8, identified in FIG. 2 as theloop oscillator, while the curve 136 represents the voltage output fromoscillator 20, identified as the internal oscillator. A voltmeter shownat 110 in FIG. 1 is conveniently connected across resistor 50, eitherpermanently, or temporarily during the tuning of the system, and thevoltage thereon will be zero with the in phase condition of theoscillators.

' Starting with the tank circuits tuned to the same natural resonantfrequency, the inductance of coil 25 is next adjusted to increase thenatural resonant frequency of the tank circuit of internal oscillator 20to provide a predetermined voltage across resistor 50, which may be readon voltmeter 110. In a typical system properly tuned and adjusted, thisvoltage will be 10 volts. Since the oscillators 8 and 20 drive eachother into synchronization through the common load or coupling resistor29, the result of such tuning of the tank circuit of oscillator 20 is tocause the output signal therefrom as applied to the emitter oftransistor 35 to be advanced in phase with respect to the phase of thevoltage from the loop oscillator, as applied to base 34. During theperiod that the base 34 is positive, curve 144, with respect to theemitter, as represented by curve 146 in the section of FIG. 2 titledTuned for Operation, that is, for the time T1, the output voltage atcollector 42 becomes less positive. These recurring less positive pipsestablish a potential across storage capacitor 54 representing theambient condition.

As heretofore pointed out, the tank circuit of the internal oscillator20 is adjusted to have a resonant frequency higher than the frequency ofthe loop oscillator 8 in the absence of an external object over theloop. An external object in the presence of the loop tends to reduce theinductance thereof and thereby increase the resonant frequency of thetank circuit of the loop oscillator. The oscillators are sointer-connected that they drive each other to be and remain locked insynchronization as to frequency but the phase of the loop oscillatorsignal lags that of the internal oscillator. Accordingly, the base oftransistor 35 is positive with respect to its emitter for time T1 asshown in FIG. 2. With the device properly tuned in the absence of anobject over the loop the integrated voltage across resistor 50 istypically 10 volts.

The signals corresponding to the presence of an automobile are shown inthe curves over the legend Vehicle Detected in FIG. 2. The looposcillator signal 154 has decreased in phase difference with respect tothe internal oscillator signal 156 and now the base of transistor 35 ispositive with respect to the emitter for the shorter time T2, and theintegrated voltage produced across resistor 50 is lower, and typicallyapproximately 7 volts, whereby a reverse bias is applied is applied tothe base of transistor 56 through capacitor 54, cuts off transistor 56,thereby reducing the potential on collector 64, resulting in a reductionin the voltage on the base of transistor 60 and on the base oftransistor 74, causing these transistors also to cut off. The charge onthe memory capacitor 54 can only leak off through leak resistor 57.Under these conditions, the capacitor 54 may discharge within about 30minutes sufficiently to cause transistor 56 to begin to conduct.Immediately the potential of collector 64 begins to rise increasing thepotential on the base of transistor 60 which, also, begins to conductfurther increasing the potential on the base of transistor 56.Transistors 60 and 56, accordingly, regeneratively become fullyconductive, and the increased potential of the collector of transistor56 causes transistors 74 and to become conductive. It will be apparentthat the length of time after the vehicle has arrived over the loop, andafter the voltage across resistor 50 has been reduced, during whichtransistor 56 remains reverse biased is dependent upon the value ofresistor 57 and, of course, upon the value of the memory capacitor 54.At any time that the external object removes from the location of theloop 1, the tuning of the tank circuit of oscillator 8 is affected toreestablish the conditions represented by the Tuned for Operationcurves, with the voltage across resistor 50 re-established at 10 volts.The signal applied to capacitor 54 upon leaving of a vehicle is in thedirection to tend to increase conductivity of transistors 56 and 60,whereby the charge on capacitor 54 is quickly re-established in accordwith the ambient inductance of loop 1.

Gradual changes which may occur in the inductance of loop 1, as a resultof changing temperatures or the occurrence of rain, do not occur withsufficient rapidity to cut off transistors 56 and 60, whereby the chargeon capacitor 54 continuously adjusts to the ambient conditions throughthe short time constant circuit through resistors 58 and 61 and thecollector-emitter circuit of transistor 60.

lclaim:

1. In an electronic presence detector including first and secondoscillators, means maintaining said oscillators in frequencysynchronization, said oscillators being tuned to slightly differentfrequencies thereby to provide output signals out of phase with eachother, means responsive to the presence of an external object to changethe phase difference between the output signals of said oscillators,means to limit the amplitudes of said output signals, a phase detectorconnected to receive said limited output signals, and indicator meansresponsive to the output of said phase detector, characterized in thatsaid phase detector comprises a transistor having one said output signalapplied to its base and the other to its emitter and having itscollector connected to a load impedance, said indicator means beingoperatively connected to respond to the signal voltage on saidcollector.

2. In an electronic presence detector including first and secondoscillators operating at the same predetermined frequency, meansresponsive to the presence of an external object to advance or retardthe phase of the output signal of one of said oscillators with respectto the phase of the output signal of the other of said oscillators,means to limit the amplitudes of said output signals, a phase detectorconnected to receive said limited output signals, and indicator meansresponsive to the output of said phase detector, characterized in thatsaid phase detector comprises a transistor having one of said oscillatoroutput signals applied to its base and the other of said oscillatoroutput signals applied to its emitter and having its collector connectedto a load impedance, said indicator means being operatively connected torespond to the signal voltage on said collector.

3. The combination according to claim 2 wherein the maximum phasedifference between said oscillator output signals is less than degrees.

4. The combination according to claim 3 wherein said oscillators aretuned to different frequencies in the absence of an external object andare provided with feedback means responsive to the phase differencebetween the respective output signals thereof to maintai fre u no snchronizatio of said oscillators.

Ari efecti' or irc presence etector comprising an mductance loopdisposed at a location at which presence of an external object is to bedetected, oscillator and detector means including said loop providing anoutput signal of which the voltage amplitude is responsive to theinductance of said loop and decreases in response to increase of saidinductance, a memory capacitor, low impedance means to charge saidcapacitor to the voltage of said signal, a high resistance leak resistorfor said capacitor establishing a long time constant direct currentdischarge circuit therefor, a second direct current discharge circuitfor said capacitor comprising impedance means and a first currentcontrol device having a control electrode, the impedance of said seconddischarge circuit being controlled by the potential on said electrodebetween a maximum impedance greater than and a minimum impedance lessthan the impedance of said resistor, a second current control deviceprovided with self-biasing means and having a control electrodeconnected through said memory capacitor to said charging means, saidsecond current control device being driven to cut off in response toabrupt reduction of said signal voltage, means connecting said controlelectrode of said first current control device to said biasing means tocut off said first current control device in response to cut off of saidsecond current control device, said current control devices beingnormally conductive, whereby said memory capacitor has a short timeconstant and its charge follows said voltage through changes in theinductance of said loop occurring gradually as a result of ambienttemperature changes, but has a long discharge time constant and itscharge does not follow said voltage through abrupt decreases thereofresulting from rapid changes in said inductance occurring upon thearrival of an object at the loop location, and whereby, following aperiod of slow decay of the charge on said capacitor through saidresistor initiating conduction of said second current control device,said control devices regener'atively resume fully conductive conditionto thereby tune out said object and condition the detector to respond tofurther external objects, and indicating means responsive to thecondition of one of said current control devices.

1. In an electronic presence detector including first and secondoscillators, means maintaining said oscillators in frequencysynchronization, said oscillators being tuned to slightly differentfrequencies thereby to provide output signals out of phase with eachother, means responsive to the presence of an external object to changethe phase difference between the output signals of said oscillators,means to limit the amplitudes of said output signals, a phase detectorconnected to receive said limited output signals, and indicator meansresponsive to the output of said phase detector, characterized in thatsaid phase detector comprises a transistor having one said output signalapplied to its base and the other to its emitter and having itscollector connected to a load impedance, said indicator means beingoperatively connected to respond to the signal voltage on saidcollector.
 2. In an electronic presence detector including first andsecond oscillators operating at the same predetermined frequency, meansresponsive to the presence of an external object to advance or retardthe phase of the output signal of one of said oscillators with respectto the phase of the output signal of the other of said oscillators,means to limit the amplitudes of said output signals, a phase detectorconnected to receive said limited output signals, and indicator meansresponsive to the output of said phase detector, characterized in thatsaid phase detector comprises a transistor having one of said oscillatoroutput signals applied to its base and the other of said oscillatoroutput signals applied to its emitter and having its collector connectedto a load impedance, said indicator means being operatively connected torespond to the signal voltage on said collector.
 3. The combinationaccording to claim 2 wherein the maximum phase difference between saidoscillator output signals is less than 90 degrees.
 4. The combinationaccording to claim 3 wherein said oscillators are tuned to differentfrequencies in the absence of an external object and are provided withfeedback means responsive to the phase difference between the respectiveoutput signals thereof to maintain frequency synchronization of saidoscillators.
 5. An electronic presence detector comprising an inductanceloop disposed at a location at which presence of an external object isto be detected, oscillator and detector means including said loopproviding an output signal of which the voltage amplitude is responsiveto the inductance of said loop and decreases in response to increase ofsaid inductance, a memory capacitor, low impedance means to charge saidcapacitor to the voltage of said signal, a high resistance leak resistorfor said capacitor establishing a long time constant direct currentdischarge circuit therefor, a second direct current discharge circuitfor said capacitor comprising impedance means and a first currentcontrol device having a control electrode, the impedance of said seconddischarge circuit being controlled by the potential on said electrodebetween a maximum impedance greater than and a minimum impedance lessthan the impedance of said resistor, a second current control deviceprovided with self-biasing means and having a control electrodeconnected through said memory capacitor to said charging means, saidsecond current control device being driven to cut off in response toabrupt reduction of said signal voltage, means connecting said controlelectrode of said first current control device to said biasing meAns tocut off said first current control device in response to cut off of saidsecond current control device, said current control devices beingnormally conductive, whereby said memory capacitor has a short timeconstant and its charge follows said voltage through changes in theinductance of said loop occurring gradually as a result of ambienttemperature changes, but has a long discharge time constant and itscharge does not follow said voltage through abrupt decreases thereofresulting from rapid changes in said inductance occurring upon thearrival of an object at the loop location, and whereby, following aperiod of slow decay of the charge on said capacitor through saidresistor initiating conduction of said second current control device,said control devices regeneratively resume fully conductive condition tothereby tune out said object and condition the detector to respond tofurther external objects, and indicating means responsive to thecondition of one of said current control devices.